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Comparison of Mn doped GaAs, ZnSe, and LiZnAs dilute magnetic semiconductors

Comparison of Mn doped GaAs, ZnSe, and LiZnAs dilute magnetic semiconductors. J.Mašek, J. Kudrnovsk ý , F. M á ca, and T. Jungwirth. Introduction: A II B VI vs. A III B V DMS New proposition: Li(Zn,Mn)As Basic picture: scaling of Tc in (Ga,Mn)As

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Comparison of Mn doped GaAs, ZnSe, and LiZnAs dilute magnetic semiconductors

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  1. Comparison of Mn doped GaAs, ZnSe, and LiZnAs dilute magnetic semiconductors J.Mašek, J. Kudrnovský, F. Máca, and T. Jungwirth FZU 30.5.06

  2. Introduction: AIIBVI vs. AIIIBV DMS • New proposition: Li(Zn,Mn)As • Basic picture: scaling of Tc in (Ga,Mn)As • Comparison of (Zn,Mn)Se, (Ga,Mn)As, and Li(Zn,Mn)As based on density functional calculations • Li(Zn,Mn)As – a detailed study • Conclusions FZU 30.5.06

  3. (AII,Mn)BVI DMS • (Cd,Mn)Te, (Zn,Mn)Se • Isovalent substitution of Mn(AII) • Mn atoms neutral, with spin 5/2 • Wide concentration range of alloys • Intrinsic semiconductors (may not be the case for other transition metals) FZU 30.5.06

  4. (AII,Mn)BVI DMS … cont. • Giant AF splitting of the valence band: Ev(↑)>Ev(↓) • Antiferromagnetic exchange coupling of local moments (superexchange) • Complicated phase diagram due to frustration … semimagnetic semicond. • Ferromagnetic state achieved only in strongly n-type doped materials FZU 30.5.06

  5. (AIII,Mn)BV DMS • (Ga,Mn)As, (Ga,Mn)N • Non-isovalent substitution of Mn(AIII) • Limited range of alloying (x<0.1) • Mn acts as single acceptor • Without extra doping, (Ga,Mn)As is p-type semiconductor (EF ~ Ev – 0.2eV) • Local moment 5/2 + hole spin -1/2 FZU 30.5.06

  6. (AIII,Mn)BV DMS … cont. • Strong ferromagnetic RKKY interaction of local moments due to free carriers • Ferromagnetic state with Tc<170K (Tc increases with both x and h) • Superexchange seems unimportant and is usually neglected • Troubles with selfcompensation (Mn(int)) FZU 30.5.06

  7. Open problems • Limitationsof Tc • Dependence of Tc on x and density of carriers from the microscopic theory • The role of superexchange in (Ga,Mn)As • Possibility of n-type FM semiconductors • Optimization of the host materials FZU 30.5.06

  8. Proposition: Li(Zn,Mn)As • Band structure of LiZnAs similar to ZnSe and GaAs • a(LiZnAs)= 5.815Å, a(GaAs)= 5.642 Å → hybridization • Easy substitution of Mn for Zn expected • Number of carriers related to non-stoichiometry in Li sublattice (vac(Li)=acceptor, Li(int)=donor) FZU 30.5.06

  9. Basic picture • sp3 bands + Anderson Hamiltonian for 3d5-states of Mn • Spin-orbit interaction neglected • CPA description of the mixed crystal in the FM state • Erev≡ energy of flipping a single atomic moment • In mean-field approximation, Tc ~ Erev . FZU 30.5.06

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  11. Preliminary results for (Ga,Mn)As • General scaling rule: Tc/x≈ f(n/x) • Tc<0 for |n|<x/4 … superexchange important ! • Saturation of Tc for n ≈ -x • FM state (Tc>0) possible for both p-type and n-type ! FZU 30.5.06

  12. Detailed density-functional study • LMTO-CPA, LDA (partly LDA+U) • Supporting calculations: FP LAPW • Densities of states & distribution of Mn d-states in (Zn,Mn)Se, (Ga,Mn)As, and Li(Zn,Mn)As • Mean-field Tc in rigid-band approximation • Tc in realistic, co-doped DMS FZU 30.5.06

  13. DOS: (Ga,Mn)As vs. (Zn,Mn)As FZU 30.5.06

  14. DOS: (Zn,Mn)Se vs. Li(Zn,Mn)As FZU 30.5.06

  15. Curie temperature: rigid-bandapprox. FZU 30.5.06

  16. Curie temperature: co-doped DMS FZU 30.5.06

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  18. Summary • Dependence of Tc(n)similar in all (Ga,Mn)As, (Zn,Mn)Se, and Li(Zn,Mn)As: • no difference of II-VI and III-V DMS • Superexchange→ unstable FM state for low concentration of carriers • n-type ferromagnetic DMS available at high concentration of donors • Seems realistic only in Li(Zn,Mn)As FZU 30.5.06

  19. Li(Zn,Mn)As – more details • Problems: • Microscopic nature of Li non-stoichiometry (vac(Li) and Li(int), Li(Zn) and Zn(Li) ) • Limits of solubility of Mn in LiZnAs • Mn solubility in non-stoichiometric material FZU 30.5.06

  20. Substitution of Mn for Zn not affected by non-stoichiometry FZU 30.5.06

  21. Large formation energies of vac(Li) and Zn(Li)→substoichoimetry unprobable FZU 30.5.06

  22. Both Li(int) and Li(Zn) possible, leading to either n- or p-type material FZU 30.5.06

  23. Dynamical equilibrium • Principal defects in Li(Zn,Mn)As: Mn(Zn), Li(Zn),and Li(int) • Formation energiesEs,i (xs,xi)of Li(Zn) and Li(int) as functions of partial concentrationsxsandxi . • Balanced state:Es(xs,xi) = Ei(xs,xi) . FZU 30.5.06

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  25. Partial concentrations of Li(Zn) and Li(int) FZU 30.5.06

  26. Conclusions • Both p-type and n-type ferromagnetic DMS possible with suitable doping of III-V, II-VI, and I-II-V DMS • AFM exchange in compensated materials • Electron-mediated FM: disorder-induced reconstruction of CB + partial occuparion of side minima +strong admixture of d-states (unclear) • Li-rich Li(Zn,Mn)As seems a good candidate for n-type FM semiconductor. FZU 30.5.06

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